1. Field of the Invention
This invention relates generally to semiconductor circuits for generating pulse currents and more particularly to a thyristor pulse generating circuit having means for preventing high temperatures at the hot spot of the thyristor and for improving the di/dt thereof.
2. Description of the Prior Art
When a thyristor having a low current-rising coefficient di/dt is triggered to pass a current having a high current-rising coefficient, the current is initially concentrated to pass on a semiconductor region. Accordingly, the region may be heated to a high temperature. In general, the high temperature heated part is referred to as a hot spot. When the temperature of the hot spot is increased too high, the thyristor may be damaged, the limit being provided by the di/dt rating. In a conventional circuit having such a thyristor, in order to improve the di/dt; a saturable reactor 5 has been inserted in series with a thyristor 6 as shown in FIG. 1a.
The effect of the saturable reactor is to permit passing of only the exciting current component of the saturable reactor through the thyristor by applying all of the voltage of the circuit to the saturable reactor until saturation of the saturable reactor even though the thyristor is triggered. In such a manner it was believed that the effect of the saturable reactor was to improve the thyristor di/dt by enlarging the initial turn-on region of the thyristor and by decreasing the switching loss.
While somewhat satisfactory, problems exist in the conventional circuit in that the exciting current component of the saturable reactor which is passed to the thyristor is small during the period for saturating of the saturable reactor. Accordingly, even though the saturable reactor is inserted, the saturable reactor is not very effective for enlarging the initial turn-on region and decreasing the switching loss. Thus, the temperature at the thyristor hot spot is not reduced and the di/dt of the thyristor is not effectively improved.